Memories may include memory cells that need to be periodically “refreshed” in order to retain the data stored. For example, with conventional volatile memory such as dynamic random access memory (DRAM), the memory cells need to be refreshed to accurately store data. In memories such as these, circuitry may be included that provide internal refresh addresses that are used to refresh the cells. As the refresh addresses are generated, the addresses are decoded and the memory corresponding to the addresses are accessed and refreshed.
For a memory to pass production testing, it must meet minimum data retention times, that is, be able to accurately store data for a minimum time. The minimum time is typically related to the maximum time the memory can go without being refreshed, according to a specification provided by the memory manufacturer. Where the maximum refresh time is longer, the minimum data retention time must be longer as well. Conversely, where refresh can occur more frequently (i.e., shorter maximum refresh time) the data retention time can be shorter.
Data retention times for the memory cells may vary due to process variations during fabrication of the memory. As a result, some memory cells may be able to store data for a longer time without being refreshed than other memory cells of the memory. In order to meet a data retention time requirement, the memory cells having the shortest data retention times, typically a minority of the overall number of memory cells, must satisfy the requirement. A memory having even a low number memory cells that cannot meet the requirement may be rejected at testing although the majority of the memory cells can meet the requirement.
Therefore, it may be desirable for a memory to refresh some memory cells more frequently than others, for example, refreshing memory cells that have relatively shorter data retention times.